[link|https://elektrotanya.com/panasonic_sa-pm_17_.pdf/download.html|This|new_window=true] is the service manual.
This is the standby control circuit.
[image|3292491]
My first guess is the relay on the lower right is stuck, or the control circuit has a malfunction and is holding the relay closed.
Q575 turns it off/on and is controlled directly by the micro. Standby/SysV6 circuitry on the micro schematic sheet may also be involved.